THE SEASIDE promenade in Douglas, on the Isle of Man, a self-governing British Crown dependency, boasts grand Victorian buildings and a horse-drawn tram. Once they helped it to compete with the likes of Llandudno and Blackpool for the tourist masses of England’s north-west. When cheap air travel meant these holiday towns were abandoned, most fell into disrepair. But Douglas reinvented itself as an offshore financial centre. Today finance provides over a third of the island’s GDP, of which around half is from insurance. Now new transparency rules put that at risk.

From January 1st the island’s insurers will have to be more open with clients, in particular on the subject of brokers’ commissions. Britain has had similar rules since 2013. International organisations such as the OECD, a club of rich countries, and the Financial Action Task Force, an intergovernmental anti-money-...